This invention relates generally to multiplexing systems, and, more particularly, to an improved optical multiplexer/demultiplexer.
In practice, the need frequently arises for communications or reconnaissance systems which simultaneously convey multiple messages from a large number of information sources in one location to a large number of users at another location. Multiplexing systems economically meet this need by combining the messages from several information sources, which are then transmitted as a composite group over a single transmission facility, with provision at the receiver for separation (demultiplexing) back to individual messages.
In recent years with the development and implementation of fiber optic technology into practical transmission systems a great deal of attention has been given to a multiple carrier technique referred to as wavelength division multiplexing (WDM). This technique, which is the optical equivalent of frequency division multiplexing technique employed in RF coaxial transmission networks, can be used to increase the information transfer capacity of the medium. In the wavelength division multiplexing technique each discrete data channel is modulated onto an optical carrier of a fixed wavelength. Each of the individual carriers are then superimposed onto the optical transmission medium. At the optical receiver the individual carrier must be reestablished by filtering the composite carrier into its individual wavelength components.
One of the parameters that encourages the development of wavelength division multiplexing is the inherent narrow spectral characteristics of optical sources. The basic problem in developing this technique is the broad spectral response of the available photodetector unit. In order to successfully employ these photodetector units in a wavelength division multiplexing system the photodetector must be preceded by some form of selective optical filtering.
To achieve the desired optical filtering to make wavelength division multiplexing practical, a series of optical techniques have been examined. Some of these techniques employ prisms, blazed plane reflection gratings, simple thick reflection gratings, multiple thick reflection gratings, simple thick transmission gratings and dicromatic coatings. An article of particular interest which describes and compares these techniques is "Wavelength Multiplexing in Multimode Optical Fibers", by W. J. Tomlinson, Applied Optics, Vol. 16, No. 8, ppgs 2180-2194, August 1977.
Unfortunately, size is a critical requirement when providing selective optical filtering. To date, however, multiplexing systems of the past have been either unreliable in complete separation of the individual wavelength components or are of such a large size as to provide an overall system which is incompatible with the desired small size requirements.